1. Field of the Invention
The present invention pertains to the art of microwave cooking appliances and, more particularly, to a microwave cooking appliance including multiple magnetrons, each emitting a respective microwave energy field, and an electronic controller for establishing relative phase angles for the microwave energy fields.
2. Discussion of the Prior Art
Cooking appliances utilizing a directed microwave energy field to cook a food item have existed for some time. In general, a cooking process is performed by heating the food item by directing a standing microwave energy field into an oven cavity such that the microwave energy field reflects about the oven cavity and impinges upon the food item. As the microwave energy field impinges upon the food item, the field is converted into heat through two mechanisms. The first heating mechanism is caused by the linear acceleration of ions, generally in the form of salts present within the food item. The second is the molecular excitation of polar molecules, primarily water, present within the food item. However, the nature of the standing waves results in localized areas of high and low energy which cause the food to cook unevenly. This is especially true in larger ovens where the size of the cavity requires a more uniform energy distribution in order to properly cook the food. To attain an even or uniform energy distribution, the microwave energy must be introduced into the oven cavity in a manner which creates a constructive standing wave front that propagates about the oven cavity in a random fashion.
Various methods of directing microwaves into cooking chambers to minimize hot and cold areas within a food item have been proposed in the prior art. These methods range from altering the pattern of the standing waves by varying the frequency of the microwave energy field, to incorporating a stationary mode stirrer which simulates a change in the geometric space of the cooking chamber. Methods of changing the wave pattern also include the incorporation of a rotating blade stirrer which functions to reflect microwave energy into a cooking cavity in various patterns. Traditionally, stirrers have been located at various points in the microwave feed system, ranging from adjacent to a microwave energy source, to a position within the cooking chamber itself. Some stirrers include various openings which are provided to disperse the standing waves, and others have various surface configurations designed to reflect the standing waves. Stirrers are either driven by a motor or by air currents supplied by a blower. In any event, all of these methods share a common theme, i.e., to reflect and/or deflect the microwave energy into a cooking cavity such that a uniform distribution of standing wave patterns can be achieved.
Other methods of modifying the standing wave patterns include altering the design of a waveguide or path through which the microwave energy field is introduced into the oven cavity. Prior art waveguide designs include cylinders, square boxes, and a variety of other configurations designed to cause the standing waves to interfere with one another such that the wave pattern was randomized and maximum energy was directed into the oven cavity. Other designs have included matching the dimensions of the wave guide to the wavelengths of the standing wave pattern. However, as each waveguide is designed to meet the particular specification of each oven cavity, a new waveguide must be designed to accommodate each different model oven.
As oven cavities have grown in size and microwave technology has been combined into radiant and/or convection ovens, the uniform distribution of the standing waves has become of even greater concern. For this reason, manufacturers have modified their designs to include complex waveguides, multiple stirrers, motor driven, variable speed stirrers, and turntables, all of which were intended to enhance wave pattern distribution to a more uniform character. Certainly, the mechanisms which serve to alter the microwave energy field, e.g., stirring fans and turntables, add to the complexity of designs and introduce potential failure points, thus reducing the service life of such appliances.
Based on the above, there exists a need for a microwave delivery system which will function to establish a uniform standing wave pattern in an oven cavity in a cost efficient manner. More specifically, there exists a need to effectively enhance a developed microwave energy field within an oven cavity of a cooking appliance in order to eliminate, or at least substantially reduce, the existence of hot and cold spots without the additional requirement for mode stirring, rotating the food, or the like.
The present invention is directed to a microwave cooking appliance which efficiently cooks a food item placed within an oven cavity by shifting the phase angle of a plurality of microwave energy fields. The microwave cooking appliance of the present invention includes a plurality of magnetrons and an electronic control unit which initiates a phase angle shift between the microwave energy fields emitted by the magnetrons. Specifically, the phase angle shifting eliminates the presence of localized hot and cold spots by creating constructive standing wave fronts within the oven cavity. In accordance with the invention, the food item is cooked evenly without requiring complex waveguides, mode stirrers or turntables.
In accordance with one embodiment of the present invention, the plurality of magnetrons are arranged on opposing ends of the oven cavity. In a preferred form of the invention, each of the plurality of magnetrons are spaced at a distance equal to multiples of xc2xc xcex, where xcex is the wavelength of the particular microwave energy field. The total energy delivered to the oven cavity is the combined microwave energy fields emitted from the plurality of magnetrons. In the most preferred form of the invention, the energy from three 400 Watt magnetrons are combined to deliver a total energy field to the oven cavity in a range of between 1000-1200 Watts.
Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description of a preferred embodiment when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views.